This invention relates to a multplexer, and more particularly to a statistical time-division multiplexer suitable for the case of using a public switching network in the high-speed trunk circuit.
There has been known a statistical time-division multiplexer (will be termed "STDM" hereinafter) in which a time-division multiplexer implementing the statistical time-division multiplexing function by the intelligent function is connected between a group of data terminal units and a transmission path (high-speed trunk line), as disclosed in JP-A-59-110240 for example. FIG. 1 shows a general system configuration of the conventional STDM. In FIG. 1, data terminal equipment DTE 1-n and DTE 1'-n' are a plurality of terminal units which send and receive data by way of STDMs 20. The communication takes place between corresponding units such as between DTEs 1 and 1', 2 and 2', and n and n'. The two STDMs 20 are connected by one (or more) high-speed trunk line 22, for which a leased line or high-speed digital line is used.
FIG. 2 shows the internal blocks of the STDM 20. In FIG. 2, data sent from each DTE through low-speed lines 21 are received by a low-speed line interface 12 and stored in a data buffer 13. A multiplexing unit 14 multiplexes the contents of the data buffer 13 into frames each formed of 1 to several bytes of data sent from each DTE, and sends them out by way of a high-speed trunk line interface 15. Data received over the high-speed trunk line 22 are demultiplexed into data for each DTE by the multiplexing unit 14, and delivered to each DTE by way of the data buffer 13 and low-speed line interface 12. Data within a frame are set in such an order that the originating DTEs of data may be identified respectively. Generally, data sent from a DTE is followed by a pause before sending the successive data caused by the communication protocol, resulting in an effectively lower data rate than the nominal transmission speed of the low-speed line, and on this account, by transferring data of one DTE while other sending DTE does not send data, the total transmission rate of all the low-speed lines 21 can be nominally higher than that of the high-speed trunk line 22.
As mentioned above, the STDM utilizes the pauses in data transmission of each DTE to send other data and total data rate of the low-speed lines can be higher than the high-speed trunk line. However, in actual systems the total amount of data per unit time from all DTEs exceeds the speed of high-speed trunk line in some situations. In such a case, if the average traffic quantity of low-speed line data is below the capacity of the high-speed line speed, communications between the sending and receiving DTEs take place normally, although the transmission delay will increase. However, if this situation lasts a long time, the data buffer will overflow, resulting in the loss of data and the block of communication between the sending and receiving DTEs.
Generally, the ratio of the total speed of low-speed lines to the speed of high-speed trunk line is determined in consideration of traffic congestion from low-speed lines, so that the data buffer overflow can be avoided at the time of maximum traffic.
Even for a system, in which traffic is intensive so as; to form high peaks in certain narrow time bands in a day but the average daily traffic is low, it is necessary to determine the number of high-speed trunk lines (speed) necessary to meet the peak traffic. Consequently, the nature of the higher total data rate of low-speed lines than the rate of the high-speed trunk line, which is an advantage of STDM, cannot fully be practiced, leaving the high-speed trunk lines to work in a lower duty cycle.